In the past, a mesoporous substance with mesopores having a pore diameter of 2 to 50 nm has been synthesized from a silica based material using the property of a certain surfactant or the like which forms a micellar aggregate by self-organization in a solution and using it as the template. In 1992, with a surfactant as the template, a silica porous body with mesopores having a diameter of not less than 2 nm has been developed by Mobil Corporation (Non-Patent Document 1). Non-Patent Document 1 discloses a method of synthesizing two types of mesoporous silica, such as MCM-41 type in which cylindrical pores having a diameter of 2 to 8 nm form a two-dimensional hexagonal structure, and MCM-48 type in which pores form a three-dimensional cubic structure, by the reaction with a silica component using cethyltrimethyl ammonium bromide (CTAB) as the template.
In addition, there has been disclosed a method for producing a mesoporous silica (SBA-15) having a two-dimensional hexagonal structure and having a pore diameter of not less than 10 nm by the reaction with a triblock copolymer surfactant, Pluronic P123 (EOmPOnEOm, m=17, n=56, BASF), which is composed of hydrophilic ethylene oxide (EO) and hydrophobic propylene oxide (PO) used as the template (Non-Patent Document 2). Furthermore, formation of a mesoporous material from a crystalline metal oxide such as titania or the like having a crystalline structure has been attempted. However, when heating is conducted to change from a material composed of an amorphous material to a material having a crystalline structure, a wall film in a mesoporous material is very thin, so that pores are collapsed without keeping an amorphous mesoporous shape.
As a method of obtaining a non-silica based material, there has been disclosed a method of producing a titania thin film having both an anatase crystallite and a nanosized columnar structure by forming a titania thin film having a three-dimensional hexagonal structure in which the surfactant Pluronic P123 is present in mesopores, followed by burning it (Non-Patent Document 3). There has been disclosed a technology for forming mesoporous particles by the same method (Patent Documents 1 and 2).
Meanwhile, the following applications have been studied.
In late years, with the progress of miniaturization, high integration and high density in semiconductor elements or multi-layer wiring boards, an insulating material having a low dielectric constant contributing to shortening of the signal propagation delay time has been in demand. The relative dielectric constants of a silicon oxide film, a silicon nitride film and a polyimide resin, which have been widely used at present, are respectively about 4 to 5, 7 to 9, and 3.5 to 4. On the other hand, it has also been studied to produce a low dielectric constant material by forming a porous body (mesoporous silica) from a silica precursor solution and a surfactant. This porous body has been disclosed in Non-Patent Documents 1 and 2.
On the other hand, in recent years, with the progress of large volumes of information and higher speed of information processing, miniaturization, high integration, high speed and power saving in electronic circuits have been in demand, and an insulating material having a low dielectric constant contributing to shortening of the signal propagation delay time has been in demand. The relative dielectric constants of a silicon oxide film, a polyimide resin and a fluorine-containing resin, which have been widely used at present, are respectively about 4 to 5, 3.5 to 4, and 2 to 2.5. Furthermore, in order to reduce the dielectric constant of a film, the dielectric constant may be reduced by introducing avoid inside the film. But, at this time, there is a problem of deterioration in the film strength. A method of forming a coating film formed by dispersing hollow silica particles in a resin matrix, and applying it is also considered (Patent Document 3).
An optical material having a low refractive index is applied to an antireflection film, an optical waveguide, a lens, a prism and the like, and it is used for an anti-glare treatment which suppresses reflection from a display surface, cladding of an optical waveguide and the like. In the past, as a material having a low refractive index, there are a fluorine compound (refractive index: 1.34) such as Cytop (a product of Asahi Kasei Corporation) or the like, a compound such as magnesium fluoride (refractive index: 1.38) or the like, and a compound formed by dispersing ultrafine particles thereof in a resin or the like (Patent Documents 4 and 5).
For the purpose of improvement of reduction in weight and thermal insulation performance, a hollow glass (glass balloon) has been used as a filler in many cases. However, a material having a porosity of not less than 70% and a diameter of about 100 μm is generally used as a hollow glass, but there has been a problem of causing cracks during mixing with a resin because a thickness of the glass wall becomes thin. On the other hand, in recent years, a silica gel foam or the like has been used in many cases (Patent Documents 6 and 7).